Polymers and copolymers crosslinked by free radical processes, including through use of organic peroxides and/or azo initiators are known to have superior properties, particularly compared to polymers crosslinked by sulfur cure. These properties include high heat aging resistance, low compression set, decreased staining of metal or coated metal sheet, and easy production of colored products with enhanced color stability. In view of these beneficial properties, peroxide cure has a great deal of practical importance. A possible drawback of peroxide cure is that air must typically be excluded from the surface of a material during cure; if the air is not excluded, a tacky surface may result, due to cure inhibition by oxygen.
In many cases, manufacturers would like to switch from sulfur to peroxide cure and use existing hot air ovens; however, curing with conventional peroxide systems under these circumstances would not be viable, as a tacky surface would result. In order to avoid tacky surfaces on objects fabricated using such free radical crosslinking by organic peroxides and/or azo initiators, it has been conventional to exclude air from contact with the surface during cure. Measures to exclude oxygen add to the cost and complexity of the cure step and it is often difficult to assure the complete exhaustion of air and oxygen; for example, in steam autoclaves and in the interior of hoses. Another issue occurs when peroxides are chosen for an elastomer cure, but incomplete evacuation of the mold is achieved (e.g., during compression molding, injection molding or transfer molding). Even trace amounts of oxygen may cause significant mold fouling that necessitates frequent mold cleaning and adds cost to the operation.
In order to reduce the cost and complexity of the cure step, various methods have been suggested for preventing surface cure inhibition by oxygen during free radical crosslinking. These methods have, for various reasons, met with little or no success. In particular, none have provided a tack-free surface while providing the desirable physical properties of peroxide cure, such as superior compression. Moreover, various methods involving sulfur cure and peroxide cure are limited to unsaturated elastomers.
Further information is described in U.S. Pat. Nos. 5,001,185; 6,620,871; 6,747,099; 7,211,611 and U.S. pub. No. U.S. 2003/0096904 corresponding to PCT/JP01/06375.
Thus, it is desirable to have organic peroxide formulations and methods which cure commercially available crosslinkable elastomers and polymers, both saturated and unsaturated, in the full or partial presence of atmospheric oxygen. It is also desirable to have moldable elastomeric compositions that do not stick to the mold.